The present invention relates to an optical transmission apparatus and compensating method with capabilities of effective compensation for the chromatic dispersion of optical fibers as transmission lines, and of avoiding the dispersion compensation method alteration and adjustment when the relay unit is upgraded to the optical add drop multiplexer (abbreviated as OADM) in the wavelength-division multiplexing optical transmission system.
As means for a large transmission capacity of optical transmission system, the wavelength-division multiplexing (abbreviated WDM) transmission system is practically used in which a single optical fiber is used to transmit a plurality of optical signals with different wavelengths. In addition, an optical fiber amplifier such as an erbium-doped fiber amplifier (hereafter, referred to as optical amplifier) exhibits a characteristic of amplifying in block a plurality of optical signals over a wide range of wavelengths. Therefore, a combination of the WDM and optical amplifier can achieve the simultaneous amplification of a plurality of optical different-wavelength signals, with the result that a large-capacity, long-distance transmission can be realized economically and with simple construction.
However, the optical fiber as a transmission line has a chromatic dispersion characteristic under which the wavelengths of light propagating within the fiber transmit with different velocities. It follows that the signal waveforms deteriorate as the light propagates in the optical fiber. Therefore, a dispersion compensation fiber having a dispersion value opposite to that of the transmission line is introduced to combine with the transmission line, thereby reducing the effect of the chromatic dispersion of the transmission line. Thus, this technique reshapes the deteriorated signal waveforms to be correct waveforms.
Recently, demand has increased to change from the simple communication type in which communication is simply made between two points as represented by the point-to-point system. One approach would use a bus-shape OADM mode, in which communication is made between a plurality of points with a plurality of branches and inserts provided between the two opposite points, as represented by the bus-shape system, so that the optical signals can be transmitted therein as they are. Another approach would use a ring-shape OADM mode, in which communication is made between a plurality of points connected by branches and inserts in a ring shape, as represented by the ring-shape system, so that the optical signals can be transmitted therein as they are.
In this OADM mode, too, the dispersion compensation is extremely important in order to achieve an excellent transmission characteristic. Thus, the dispersion compensation technique is expected to be simpler and more excellent.
There is known a conventional dispersion compensation method in the wavelength-division multiplexing system. This conventional method considers the self-phase modulation effect in the relay transmission using the optical amplifier (for example, see JP-A-7-74699, the fourth to fifth items in FIG. 1.
In the wavelength-division multiplexing system of the OADM mode, a system-upgrading method is advantageous to reduce the introduction cost and increase the efficiency. In this upgrading method, the point-to-point system is built at the time of the initial introduction of the system. The OADM function is added later, with the increase of communication demand. At this time, before and after the system has been upgraded to add the OADM function, it is desired that there be no need to particularly alter or adjust other portions than the added OADM function.
However, the addition of OADM function actually causes the communication quality to degrade together with the reduction of optical signal-to-noise (S/N) ratio, and the system's performance to go down at the time of addition. In addition, the alteration of the dispersion compensation method greatly affects the system construction and network so as to change the communication quality.
In the conventional wavelength-division multiplexing system, the point-to-point system is dominantly demanded, but the bus-shape system or ring-shape system using the OADM equipment is not demanded so much. However, recently the wavelength-division multiplexing system has also been demanded to have high efficiency, and flexibility of network as the communication traffic and different kinds of data are increased, and the users have had an interest in the alteration of system construction due to the addition of OADM function. Particularly, the alteration of dispersion compensation method that has close relationship with the communication quality of the system becomes a factor of reducing the communication quality at the time of adding the OADM function, and thus it is one of the items to which utmost attention must be paid in the system construction.
In the technique described in the above-mentioned JP-A-7-74699, the waveform deterioration and timing jitter due to the nonlinear effect have been reduced by cutting to zero the total dispersion value after the points where the nonlinear effects such as the above self-phase modulation effect or mutual phase modulation effect occurred. However, this chromatic dispersion compensation method is described mainly about the application to the wavelength-division multiplexing system in the point-to-point system, but not about the wavelength-division multiplexing system having the OADM function and the upgrading to that system.